Digital systems are commonly embedded on printed circuit boards. Different integrated circuits positioned on a printed circuit board may operate at different voltages. For example, with improvements in process technology, integrated circuits use lower power supply voltages, such as 3.3 Volts or 2.5 Volts, or even lower. Integrated circuits made with these processes should remain compatible with previous generations of integrated circuits. For example, a new generation 3.3 Volt integrated circuit may need to be used on a printed circuit board with an old generation 5 Volt integrated circuit. Systems of this type are commonly referred to as mixed-voltage systems.
The 3.3 Volt integrated circuit will need to have the proper supply and input voltages for operation. In addition, the 3.3 Volt integrated circuit should supply or generate the proper output voltages for interfacing with the other integrated circuits. Proper interfacing of the integrated circuits is essential for proper functional operation. Further, proper interfacing will prevent undesirable conditions, such as overstressing the devices, avoiding possible high current or latch-up conditions, and other similar concerns, thereby improving device longevity.
Many circuit architectures in mixed-voltage systems rely upon separate noisy and quiet supply voltage schemes. For example, an I/O driver may be coupled to a noisy supply while the on-chip conversion circuitry is coupled to the quiet supply. By separating the power supplies in this fashion, the circuitry connected to the quiet power supply is isolated somewhat from switching and other types of noise present on the noisy power supply.
Sometimes it is necessary to remove a circuit from a mixed-voltage system without shutting off the power within the system. This situation is referred to as a "hot socket" condition. This situation arises frequently in systems that must be up 24 hours a day, 7 days a week. If not properly designed, a chip can interfere with the rest of the system during a hot socket condition. Problems arise because as the chip is inserted, a race condition exists between the power pins and the input/output pins. If system signals reach the chip input/outputs before the system power reaches the chip's power pins, the input/outputs may behave erratically, thereby causing a disturbance to the system. The disturbance can range from an inconsequential glitch to one that disables the system.
In view of the foregoing, it would be highly desirable to provide a mechanism to identify a hot socket condition, and thereafter isolate the output pins of the effected circuit until the hot socket condition is completed.